The present invention relates to a device for controlling hydraulic braking of a trailer hitched to a tractor, which device comprises a hydraulic circuit having a brake pipe suitable for being connected to a pressurized fluid source or to a pressure-free reservoir, i.e. a reservoir without excess pressure, via a proportional pressure reducer controlled by control means comprising an electronic control unit.
Such a device can serve for braking trailers of various types, in particular farm trailers. Trailers of the “Italian” type exist in which the brake can be activated in a parking situation (i.e. in a rest situation) through lack of pressure in a hydraulic control chamber, whereas it is deactivated in order to enable the trailer to move when a medium pressure (e.g. in the range 8 bars to 15 bars) is maintained in that chamber, and it is reactivated for service braking (i.e. braking while the trailer is moving) when the pressure reaches a much higher value (in particular in the range 15 bars to 140 bars). The parking braking also performs emergency braking which is activated naturally when an incident occurs.
Trailers of the “French” type also exist that have service braking only, that braking being controlled by increasing the pressure in a hydraulic control chamber. In a variant, certain French-type trailers are each equipped with an energy accumulation device making emergency braking possible by feeding the control chamber.
Braking control devices of this type are subjected to contradictory requirements. Firstly, they must make it possible to increase the pressure rapidly in the brake pipe, so as to obtain intense braking whenever necessary. That requires the proportional pressure reducer to be dimensioned in such a manner as to enable a large amount of fluid to flow through in a short time. In addition, it is necessary to adjust the pressure finely in the brake pipe so that, when the trailer is of the Italian type, it is possible to cause a pressure in the range 8 bars to 15 bars to prevail in said brake pipe, since such a pressure is necessary to enable the trailer to move unbraked, and make it possible to avoid the brakes being engaged, even partially, since such engagement might damage them.
An object of the present invention is to propose an improved and simple device that makes it possible to satisfy those two contradictory requirements.
This object is achieved by means of the fact that the circuit includes link solenoid-valve means suitable for connecting the proportional pressure reducer to the reservoir or, via a calibrated pressure reducer, to the pressurized fluid source.
In particular, the calibrated pressure reducer is set for a pressure lying in the range 8 bars to 15 bars, e.g. in the range 10 bars to 13 bars, making it possible to guarantee release of the brake system of a trailer of the Italian type. In which case, in order to achieve such brake release, the proportional pressure reducer is connected to the calibrated pressure reducer rather than being connected directly to the reservoir. It is then by operating said calibrated pressure reducer, i.e. by connecting it to the pressurized fluid source or to the reservoir, that it is possible to obtain the desired pressure in the brake pipe.
When hard braking is necessary, the link solenoid-valve means connect the proportional pressure reducer to the reservoir, and it is by operating the proportional pressure reducer, between the pressurized fluid source to which it is also connected and this link to the reservoir, that it is possible to have the desired pressure prevail in the brake pipe.
Advantageously, a pressure limiter is disposed on a link between the calibrated pressure reducer and the link solenoid-valve means.
This pressure limiter makes it possible to limit the peak pressure at the outlet of the calibrated pressure reducer, so as to make obtaining the desired pressure (e.g. in the range 8 bars to 15 bars) in the brake pipe more reliable.
In an advantageous embodiment, the proportional pressure reducer is a solenoid valve, controlled by the electronic control unit.
In another advantageous embodiment, the proportional pressure reducer is a hydraulically controlled valve including a control chamber suitable for being connected to a control fluid feed via a proportional pilot solenoid valve, itself controlled by the electronic control unit.
In which case, the electronic control unit controls the proportional pressure reducer indirectly, said electronic unit controlling the pilot solenoid valve which, itself, controls the proportional pressure reducer by suitably feeding the control chamber thereof.
Depending on the type of trailer, emergency braking is obtained by emptying the hydraulic control chamber of the brake (Italian-type trailer), or by feeding said control chamber to a sufficient pressure (French-type trailer).
With existing devices, specific adaptation is necessary when it is desired to hitch the tractor to a trailer of one of the above-indicated types, and when emergency braking is to be obtained. Such adaptation operations are tedious and costly, and they are detrimental to the flexibility of use of the tractor.
In a particularly advantageous variant embodiment, the invention makes it possible to obtain such emergency braking, without any specific adaptation. In particular, no adaptation of the trailer is necessary, since all of the elements of the device are provided on the tractor.
Advantageously, the circuit includes a hydraulic brake selector valve that, when in a link position in which the inlet and the outlet of the brake selector valve are interconnected, is suitable for causing a link pipe to be connected to a fluid feed or to a pressure-free enclosure, i.e. to an enclosure without excess pressure, and control solenoid-valve means for controlling the brake selector valve, which means are controlled by the electronic control unit, the control solenoid-valve means for controlling the brake selector valve being suitable for taking up a first working configuration in which they make it possible to feed a first control chamber of the brake selector valve with fluid so as to cause it to go into an isolation position in which the inlet and the outlet of the brake selector valve are isolated from each other by the brake selector valve, a second working configuration in which they make it possible for a second control chamber of the brake selector valve to be fed with fluid so as to cause it to go into said link position, and a neutral configuration in which neither of the two control chambers of the brake selector valve is fed with fluid, and in which the inlet of the brake selector valve is connected to the fluid feed; the device includes means for holding the brake selector valve in position when the control solenoid-valve means go from their first working configuration or from their second working configuration to their neutral configuration; the link pipe is connected to link valve means themselves connected to the brake pipe so as to enable said brake pipe to be fed with fluid or to be connected to the reservoir depending on whether the link pipe is connected to the fluid feed or to the pressure-free enclosure; and the device includes means for delivering to the electronic control unit information indicating whether the brake of the trailer is of a first type requiring brake release by fluid feed, or of a second type, the control unit taking that information into account in controlling the control solenoid-valve means for controlling the brake selector valve.
Advantageously, the link valve means include the hydraulically controlled proportional pressure reducer, and the link pipe is connected to a pilot pipe for piloting said pressure reducer.
In which case, for applying emergency braking to a trailer of the French type, the fluid coming from the link pipe urges the proportional pressure reducer to allow the brake pipe to be connected to the fluid source. For applying emergency braking to a trailer of the Italian type, the link pipe is not fed with fluid so that the proportional pressure reducer allows the brake pipe to be connected to the reservoir.
It is also possible to imagine having the link pipe connected to the brake pipe via selection means for selecting the higher of the pressures at the outlet of the proportional reducer and in the link pipe, so as to feed the brake pipe directly when applying emergency braking to a trailer of the French type, or so as to enable said brake pipe to be connected to the reservoir when applying emergency braking to a trailer of the Italian type.
For example, the control solenoid-valve means comprise two two-position solenoid valves or one solenoid valve suitable for taking up three positions.